1. Field of the Invention
The present invention relates to synchronization acquisition processing according to a spectrum direct sequence communication system, and more particularly, to a matched filter and correlation detection method used in preamble searching.
2. Description of the Related Art
A spread spectrum communication system (CDMA communication system) features intrusion resistance, high level of signal concealment and high frequency utilization efficiency, etc. and is used for mobile communications and space communications, etc.
When a mobile station (mobile terminal) requests a base station to start a communication, a cellular phone system based on a CDMA communication system transmits a preamble signal.
The preamble signal is a signal obtained by multiplying a known signature code (original signal) with periodicity by a common spreading code.
The signature code (original signal) is a code with a total length of 4096 symbols made up of a 16-bit basic data bit pattern appearing 256 times consecutively.
Here, 16 types of basic data bit pattern are provided. Therefore, as a result, there are also 16 types of signature code. Each signature code is used to identify a reception group and the mobile station side determines which signature code should be used as appropriate.
The mobile station that requests transmission to the base station selects any one of the above-described 16 types of signature code, spreads/modulates and sends this signature code to the base station as a preamble signal.
When a signature code (original signal) is spread/modulated, the signature code is multiplied by a common scrambling code (common spreading code).
The base station receives a spread spectrum signal in which the preamble signal is inserted, despreads it and detects the signature code (original signal). Thus, the base station recognizes the communication request from the mobile station and determines the mobile station with which communication is allowed to start.
The information on the signature code detection timing is used as the timing information to establish synchronization for communication with the mobile station with which communication is allowed to start.
The base station needs to instantaneously detect communication requests from many mobile stations that exist in a cell and determine to start a communication.
A digital matched filter (hereinafter simply referred to as “matched filter”) is used to detect a signature code.
As a method for carrying out high-speed correlation detection at the base station, a method of detecting correlation is available whereby 16 matched filters are placed in parallel, multiplications are performed on 16 types of preamble signal in parallel (that is, simultaneously), the results are integrated in the time direction and a correlation is detected for every preamble signal (that is, for every signature code).
This configuration is shown in FIG. 9. FIG. 9 shows a case where 16 correlation detectors A1 to A16 are provided and each detector detects a correlation with one of signature codes (1) to (16).
Despreading code generation sections 104a and 104b each output data strings (that is, preamble signals) obtained by multiplying signature code (1) or signature code (2) by a common spreading code as despreading codes.
However, in the case of a circuit (FIG. 9) based on a system whereby 16 matched filters are placed in parallel and correlations of 16 types of preamble signal are detected simultaneously, the circuit scale increases significantly. This is accompanied by an increase of power consumption of the circuit.
Furthermore, the matched filter may be used not only for preamble searching but also for acquisition of synchronization and synchronization follow-up or RAKE combining and path determination processing for communications with directivity.
Thus, adopting a hardware configuration specialized only for preamble searching may involve a problem that that configuration will lack flexibility in the case of performing other processing and increase unnecessary parts of the circuit.
The present invention has been implemented focused on such problems and it is an object of the present invention to simplify the configuration of the matched filter section in particular to carry out high-speed preamble searching while keeping its flexibility and at the same time reduce both the circuit scale and power consumption.